This invention relates to a configuration for retention of blades in an axial direction in a disc of a turbine engine which may in particular be used in fans at the inlet of the latter.
The concern here is damage situations which may occur when a blade breaks through an impact produced by a foreign body entering the engine or upon a fault. The blade let loose strikes its neighbor and exerts significant axial and radial stresses on it which may break it in turn.
With the device provided herein, the axial component of an impact force exerted on the blade may be absorbed. This is a special configuration involving shims for securing the blade, located between the disc and a bottom of the blade which slides in a groove, and which stops its axial movement in the groove.
The securing parts form a relatively flexible set with deformation capabilities which are exploited here, by letting the blade move in the groove up to a determined limit. Relatively significant energy is absorbed without there being any excessive force exerted on the struck blade. The deformation is reversible, i.e. the shims of the securing set return to their initial state after the impact, the blade is pushed back to its initial position, other impacts may further be absorbed, and notably other impacts subsequent to the initial impact may be absorbed during the same event.
This assembly, elastic in the axial direction, of a blade sliding in an axial or slightly oblique groove differs from known configurations, where the blade is either stiffly connected to the disc and is denied any possibility of axial travel, or is axially retained by a part capable of breaking or buckling in the case of an excessive axial force. The blade undergoing the impact is likely to break easily in the first case, and the broken or buckled part will be unable to withstand other high loads during the same event, in the second case.
Another aspect of the invention is that deformation which the securing parts may undergo, is nevertheless limited, so that they do not retain permanent deformations: a stop is added for this purpose between the blade and the disc. When displacement of the blade has been sufficient for it to reach the stop position, the mechanical system becomes much stiffer, i.e. much more significant stress increases are required for producing further displacements. The energy from the axial impact as well as stresses are then distributed among both levels of the retention configuration of the stop system, and the breaking of either one of the systems is avoided.
The first level""s retention has such a flexibility that it enables the blade to slide into the recess of the disc without any significant axial stress before intervention of the second level after reaching an imposed clearance and, at this moment, the blade is retained by including the stiffnesses and capabilities of the configuration.
The utility of the invention is not limited to the absorption of impact forces subsequent to a breaking of a neighboring blade, but to all the causes of axial stresses on the blade, such as impacts from solid bodies, birds or other objects.
It will be seen that a great number of embodiments, externally different from one another, are possible for the invention, but a general definition for the latter is a configuration for retaining a blade in a disc where the blade may substantially slide axially in the disc, comprising shims for securing the blade, characterized in that the securing parts are joined together by couples of axially directed stop surfaces and finishing either on a portion of the disc or on one of the securing parts, which is combined with the disc, wherein the securing parts or the portion of the disc are at least partly flexible, and in that there is a determined clearance in the axial direction either between a portion of the disc or a portion of a part joined to the disc, and either a portion of the blade, or a portion of a part joined to the blade, said clearance is exhausted when a determined axial stress has been exerted on the blade. In other words, it may be stated that the invention is also related to a configuration for retaining with double flexibility a blade in a disc according to axial displacements of the blade, comprising a first deformation level with significant flexibility and a second deformation level marked by disappearance of clearances between at least a chain of components of the configuration and by a reduction in flexibility combined with an increasing load on the components of the chain, without any breaking or buckling of any of the components of the chain.
The configuration may be located on only one side of the blade, or on either side, wherein advantageously the clearance is then on one side and the securing parts on the other.